山西恒山巨晶状直闪石岩的成因:来自地球化学和Sm-Nd同位素的证据  

Genesis of the megacryst orthoamphibole rock from Hengshan Mts,Shanxi Province:Evidence from geochemistry and Sm-Nd isotopic data

作　　者：钱加慧[1] 魏春景[1] 周喜文[2] 初航[3] 

机构地区：[1]北京大学地球与空间科学学院,造山带与地壳演化教育部重点实验室,北京100871 [2]中国地质科学院地质研究所,北京100037 [3]天津地质矿产研究所,天津300170

出　　处：《岩石学报》2012年第9期2819-2830,共12页Acta Petrologica Sinica

基　　金：国家基础调查计划项目;国家自然科学基金(40821002;41172055)联合资助

摘　　要：山西恒山地区的巨晶状直闪石岩由石榴石、直闪石、蓝晶石、十字石、堇青石、石英、金红石、钛铁矿和少量斜长石和普通角闪石组成,岩石SiO2含量类似于中-基性岩,但比后者更富镁铁而贫钙和碱。岩石可分为Ⅰ、Ⅱ两种类型:Ⅰ类直闪石岩含有石榴石变斑晶,并相对低镁、钛和碱,而含有较高铝、铁,其高场强和稀土元素特征类似于岛弧玄武岩,岩石的143Nd/144Nd为0.511361～0.511894,tDM(Nd)=2.39～2.60Ga,Nd同位素特征与五台杂岩中的基性变质岩一致。Ⅱ类直闪石岩富含直闪石,一般不出现石榴石,相对高镁、钛和碱,而含有较低铝和铁,高场强和稀土元素特征类似于大洋中脊拉斑玄武岩。岩相学和地球化学对比表明,本区直闪石岩的原岩是由与五台杂岩变质基性岩类似的火山岩经受海水热液蚀变的产物,这种水热蚀变引起基性岩中钙、碱丢失和镁(铁)富集,并可能形成大量绿泥石等富水矿物。直闪石岩的现有矿物组合是经历高压角闪岩相变质和接续的等温降压作用形成的,其中巨晶结构是由于这种极富流体岩石在峰期之前的前进变质过程中大量脱水所致,与这种大量脱水伴随的变质分异作用导致了直闪石岩成分的进一步改造。The megacryst orthoamphibole rocks from Hengshan Mts,Shanxi Province are comprised of orthoamphibole,kyanite,staurolite,cordierite,quartz,rutile,ilmenite and a small amount of plagioclase and calcic amphibole.They have the similar SiO2 contents to intermediate-basic rocks,but much higher MgO,total Fe2O3 and lower CaO,Na2O and K2O contents than the latter.The rocks can be grouped into two types.Type I orthoamphibole rocks contain mega-grained garnet porphyroblasts,and relatively lower MgO,TiO2,Na2O and K2O but higher Al2O3 and total Fe2O3 contents.This type are similar to IAB in HFSE and REE and contain 143Nd/144Nd=0.511361~0.511894 with tDM（Nd）=2.39~2.60Ga and similar εNd（t） values to that of the metabasic rocks in the Wutai Complex.TypeⅡorthoamphibole rocks contain a large amount of orthoamphibole without garnet,and higher MgO,TiO2,Na2O and K2O but lower Al2O3 and total Fe2O3,which are similar to MORB in HFSE and REE.According to the petrography and geochemistry,we infer that protolith of the orthoamphibole rocks may have been originated from hydrothermal alteration of basic volcanics equivalent to the mafic rocks in Wutai Complex.This hydrothermal alteration caused the depletion of Ca,Na and K and relative enrichment of Mg（Fe）,and probably resulted in formation of large amount of hydrous minerals such as chlorite.The present mineral assemblages in the orthoamphibole rocks were formed due to a high-pressure amphibolite facies metamorphism with a subsequent isothermal decompression.The megacrysts may have been resulted from prograde dehydrations of the extremely fluid-rich rocks during the pre-peak stages,and metamorphic differentiation related to these prograde dehydrations may have led to a further modification of the bulk rock compositions.

关 键 词：恒山 巨晶状直闪石岩 地球化学 SM-ND同位素 热液蚀变 

分 类 号：P597[天文地球—地球化学]